Technical Field
The exemplary and non-limiting embodiments relate generally to a robot and, more particularly, to controlling movement of a robot.
Brief Description of Prior Developments
Semiconductor processing systems utilize one or more robotic manipulators to accurately deliver silicon wafers, referred to as substrates, to process modules, metrology stations, load locks and other suitable locations. Since the substrates may not be accurately positioned in the locations from which they are picked by the robot (initial misalignment), and because the robot end-effector typically does not provide a means of mechanically centering the substrate on the robot end-effector, external sensors are often utilized to detect the edges of the substrate during motion of the robot. This may be used by a control system to adjust the motion of the robot and subsequently deliver (place) the substrate accurately; regardless of the initial misalignment.
To this end, the control system typically captures the positions of the robot axes (motors, joints) when the edges of the substrate are detected by the sensors, and uses the resulting data along with the expected radius of the substrate and the coordinates of the sensors to either (1) determine the eccentricity of the substrate and use this information to compensate for the eccentricity when completing the place operation or (2) directly adjust the end point of the robot motion to place the substrate accurately regardless of the initial misalignment of the substrate. The above type of adjustment is referred to as an Adaptive Placement System (APS).
The accuracy of the APS may be affected by mechanical imperfections, such as structural flexibilities and positioning errors in belt drives of the robotic manipulator, which may distort the relationship between the captured positions of the robot axes (motors, joints) and the corresponding actual positions of the robot end-effector. Typical structural flexibilities include a flexible frame of the drive section of the robot, flexible drive shafts (torsion and bending), flexible links, flexible joints, and elastic belts and bands, which tend to stretch under tension. Positioning errors in belt drives may result from hysteresis due to meshing errors between belt and pulley teeth.
The accuracy of the APS may be affected by temperature changes in the operating environment of the robot. Temperature changes will cause thermal expansion and contraction of the robot's components, the structural frame that the robot is attached to, and the station where the substrate is delivered to. Temperature changes may be non-uniform so that different components of the system are at different temperatures and have different amounts of relative expansion or contraction. The temperature can also vary over time with different parts of the system changing at different rates. The consequence of this thermal deformation is inaccuracy in the robot's calculation of the position of its end effector. This calculation takes as its input the positions of the motors and uses pre-programmed knowledge of the geometry of the robot to calculate the position of the end effector. If the geometry of the robot is distorted by thermal effects, this calculation will be inaccurate. This affects the system in two ways. Inaccurate geometry will affect the ability of the system to position the end effector in the expected and desired location. Additionally in a system with APS, the positions of the motors are captured when the APS sensor is tripped and these data are used to calculate the position of the wafer on the end effector. These APS position capture data will be affected by an inaccurate model of the robot's geometry, causing further inaccuracy on substrate placement.